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Bertwhistle
28 February 2017 19:03:52


 


Generally days with lower upper air humidity (like today!) are the days with deep blue skies and puffy cumulus clouds, so the UV reading between clouds will be higher.


Hazy summer days with higher humidity and a bit of general high cirrus tend to have fewer cumulus clouds.


 


This is a good link showing current and forecast upper air humidity. Dark browns (low humidity) are usually associated with deep blue skies (assuming there is no lower level cloud obscuring it of course!) 


http://www.instantweathermaps.com/GFS-php/showmap-euroupper.php?run=2017022806&var=RH&lev=300mb&hour=000


 


Originally Posted by: Rob K 


Thanks for the link Rob; what you say certainly makes sense. I can correlate the RH (roughly) from your link on different days with the max UV, other factors considered (I might try on April-Sep days, say, with 10 hours + sunshine just to create a weak control measure).


Bertie, Itchen Valley.
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Lumi-Ukko
28 February 2017 20:48:34
Clear skies and humidity of 43%, with UV at 4.8. Should be as high as 6 on Friday.

Off topic, but going to do some garden clear up between bouts of office work today as I am way behind. Hopefully not too late to put in some peas, radish, and salad greens in the shadier beds. Broad beans and squash can be sown already in the sunnier beds. Tomatoes can be bedded in 3 weeks. Raspberry and Blackberry second-year cane buds just starting to open.
Bolty
28 February 2017 23:08:08
Manchester is pretty much seeing daytime UV indices of 2 now. I usually find that the first moderate UV is around mid-March here.

Last year, I think it first happened on March 11.
Scott
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Bertwhistle
01 March 2017 18:24:21

Manchester is pretty much seeing daytime UV indices of 2 now. I usually find that the first moderate UV is around mid-March here.

Last year, I think it first happened on March 11.

Originally Posted by: Bolty 


Interesting. The 2s have come early this year too. Those quoted are for Reading- we are some way south of Reading, so mid Feb is possible.


Bertie, Itchen Valley.
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Bertwhistle
01 March 2017 18:26:12

Just under, but rounded to, 2 today.


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Lumi-Ukko
01 March 2017 19:32:42
Cracking day here. Currently 20c, humidity 30% and UV of 5.
Bertwhistle
01 March 2017 20:10:33

Cracking day here. Currently 20c, humidity 30% and UV of 5.

Originally Posted by: Lumi-Ukko 


That'll get your plants going. Still on for a 6 tomorrow?


Bertie, Itchen Valley.
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Lumi-Ukko
01 March 2017 20:39:56


 


That'll get your plants going. Still on for a 6 tomorrow?


Originally Posted by: Bertwhistle 


I haven't checked but definitely a 6 still in the forecast for Friday.  Didn't get my seeds in yesterday as composting, feeding and watering all the potted fruit trees/shrubs took a lot longer than expected. The sun still sets about 5.45pm here so not much time to do anything after work. Will attempt to get them in today to take advantage of the 24c weather and high UV forecast over the next couple of days.


I probably should really be contributing to the gardening thread.


Bertwhistle
02 March 2017 18:16:35

2.2 today- best so far.


Bertie, Itchen Valley.
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Bertwhistle
03 March 2017 17:16:02

1.8 today- cloudy


Bertie, Itchen Valley.
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Lumi-Ukko
03 March 2017 20:40:41

UV of 6 today under slightly hazy skies. 25c  and humidity of 10% whilst writing.


Bertwhistle
04 March 2017 11:51:21

Currently 1.9 with the sun popping in and out behind the clouds. Could we top this year's best- a 2.3? Doesn't compare to Lummi's, of course.


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Bertwhistle
05 March 2017 08:12:49

Actually only reached 2.0 yesterday, as cloud and in-forecasted rain spoiled the afternoon. Looking out of the window right now, it's hard to believe that 2 is also forecast for today.


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Bertwhistle
06 March 2017 19:07:30

2.1 yesterday. 2.3 today- today holds the top spot.


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Bertwhistle
07 March 2017 19:19:08

2.5 today. Thought it felt a bit special in the few moments around 1pm when I got out. In fact the chart for Reading shows 2.5 from about 1130a.m. to 1p.m. Good clear air after the rain.


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Bertwhistle
08 March 2017 18:59:54

2.1 today.


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Gusty
09 March 2017 16:24:25

I spent 45 mins in the sun at lunchtime and can confirm that I have now officially started to tan a little.


The sun felt very warm today.  


Bring it on ! 


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Bertwhistle
09 March 2017 18:19:56


I spent 45 mins in the sun at lunchtime and can confirm that I have now officially started to tan a little.


The sun felt very warm today.  


Bring it on ! 


Originally Posted by: Gusty 


Yep. 2.9 today in Reading- would be a 3 on the S. Coast. Best of the year so far and counts as moderate UV factor- no wonder you've bronzed a bit.


As you say... bring it on.


Bertie, Itchen Valley.
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Bertwhistle
10 March 2017 19:26:43

2.0 today; cloudy- that's a result!


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Bertwhistle
11 March 2017 15:20:00

2.3 reached shortly after midday; still hanging around 2 even now.


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Stormchaser
11 March 2017 17:58:00


Forgot to add- notice on some days the curve is constant, but these are not the days with the highest UV- the spikier days, suggesting cumulus or other interruptions, reach the highest values; cloud cover or air type intensifying the UV?


Originally Posted by: Bertwhistle 


The sides of cumulus clouds can reflect enough sunlight toward the surface so as to intensify the total input when the sun is also shining straight down on a location. This is why on quite cloudy days those few glimpses of sun can sometimes seem unusually intense for the time of year. The effect is most pronounced when the sun is close to, but not directly overhead; the angle of reflection is closest to straight down - so the sunlight travels through the least amount of atmosphere - without there being no reflection at all.


In the UK the sun is never exactly overhead so from around early March to late October we can potentially experience reflectively enhanced sunlight on any day with the right cloud conditions.


There is also some refraction of light through clouds - particularly high ones - that can get involved but at this point things start to get very complex, enough so that climate models struggle to even parameterise the effect, so I'll shut off my technical outburst here 


 


Oh and - I have achieved a very slight tan as of 11th March, which is as early as I've ever managed. This is thanks to several days this month with light winds, a good bit of sunshine and temps reaching the mid-teens. The March of the future? 


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Bertwhistle
12 March 2017 14:35:43

Didn't make 2 today- 1.8 best. Quite a manky old day.


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Lumi-Ukko
12 March 2017 19:54:11
Currently 6.5 here with a forecast maximum of 7.02. Sun feels really strong now day over day as we close in on the equinox and the day length increase is at a maximum. Indeed, Wednesday we are forecast 7.85 under very similar conditions.
Bertwhistle
13 March 2017 12:37:48

2.6 in Reading- must be some cloud because it's bright, strong sun here- I would gamble at least 3- typical Monday after a 2 day cloud-up at the weekend.


Bertie, Itchen Valley.
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Lumi-Ukko
13 March 2017 17:34:37
3.2 here at 10am with a 7.36 high expected. Tricky UV conditions to forecast as this close to the ocean we are experiencing intermittent haar. From now until July, it is not unusual if there is no offshore flow to have thick sea-fog (Richard in Aberdeen would love it). This is a result of the warm air in-land rising and pulling in air off the ocean to replace it. This air is humid and much much cooler so condenses really fast to form thick fog. Only a mile in-land it is bright and sunny.

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